1. Field of the Invention
This invention relates to an image transfer device for excellently transferring an electrophotographically formed visible image onto a transfer medium, and more particularly to an improved image transfer device for excellently transferring such image in superposed relationship on an unfixed transfer image.
2. Description of the Prior Art
The known methods of transferring an electrophotographically formed visible image onto a transfer medium include the roller transfer method whereby a transfer medium is brought into contact with the surface of a photosensitive medium having a visible image formed thereon and a roller is resiliently urged against the back side of the transfer medium, the field transfer method whereby a transfer medium is brought into contact with the surface of the photosensitive medium and an electrically conductive roller is urged against the back side of the transfer medium to apply a voltage thereto, and the corona transfer method whereby a transfer medium is disposed along the photosensitive medium having a visible image formed thereon and corona discharge is imparted from the back side of the transfer medium. Various construction have been proposed to the apparatus for carrying out these methods.
Particularly, in the color copying machines for forming a colored copy image on the transfer medium, three or four image transfer steps have been effected with the transfer medium wrapped about the image transfer roller so that the visible image formed on the photosensitive medium may be transferred in superposed relationship onto the transfer medium. This is based on the above-mentioned roller transfer method and when a plurality of image transfer steps are effected, there occurs a phenomenon that the portion of the transfer medium which already bears a toner image can hardly receive a subsequent toner image during the subsequent image transfer step. Also, when the transfer medium is electrostatically attracted to the roller, if the humidity is high, the transfer medium can hardly twist about the transfer roller because of the variation in resistance of the transfer medium, and if the humidity is low, an unfavorable image transfer effect is liable to occur. Thus, the roller transfer method is inconvenient in many points to obtain a final image of good quality having a good color balance.
The roller transfer method also encounters problems in the construction and use of the apparatus therefor. For example, in a copying machine using various sizes of transfer mediums, when transfer medium of small size is in use, the toner on the image-bearing drum may stick to the portions of the transfer roller which are not covered with the transfer medium and such portions of the transfer roller may stain the back side of a large-sized transfer medium when it is subsequently used, or may cause non-uniformity of the transfer field which may result in irregularity of the transfer image, which will in turn make is indispensable to provide cleaner means for the transfer roller. Moreover, the operation of such cleaner means must take place when no transfer medium is present in the machine and thus, cumbersomeness is greatly increased.